Various structures have been developed to supply r.f. fields from devices outside of a vacuum chamber to excite a gas in the chamber to a plasma state. The r.f. fields have been derived from electric field sources including capacitive electrodes, electromagnetic field sources including electron cyclotron resonators and induction, i.e. magnetic, field sources including coils. The excited plasma interacts with the workpiece to etch the workpiece or deposit materials on it. Typically, the workpiece is a semiconductor wafer having a planar circular surface.
A processor for treating workpieces with an inductively coupled planar plasma (ICP) is disclosed, inter alia, by Ogle, U.S. Pat. No. 4,948,458, commonly assigned with the present invention. The magnetic field is derived from a planar coil positioned on or adjacent a single planar dielectric window that extends in a direction generally parallel to the workpiece planar surface. In commercial devices the window is usually quartz because this material has low impurity content and provides optimum results for r.f. field coupling. The coil is connected to be responsive to an r.f. source having a frequency in the range of 1 to 100 MHz and coupled to the coil by an impedance matching network including a circuit resonant to the frequency of the source. The coil is disclosed as a planar spiral having external and internal terminals connected to be responsive to the r.f. source. The circular spiral coil disclosed by Ogle has been modified to include linear, elongated elements generally in a spiral configuration, to process workpieces having square and rectangular shapes. Coultas et al., U.S. Pat. No. 5,304,279 discloses a similar device employing permanent magnets in combination with the planar spiral coil.
Cuomo et al., U.S. Pat. No. 5,280,154 and Ogle, U.S. Pat. No. 5,277,751 disclose a variation of the aforementioned processor wherein the linear spiral coil is replaced by a solenoidal coil. The solenoidal coil is wound on a dielectric mandrel or the like and includes plural helical-like turns, a portion of which extend along the dielectric window surface. The remainder of the coil extends above the dielectric window. Opposite ends of the solenoidal coil are connected to an r.f. excitation source.
None of the prior art plasma processors with which we are familiar is well adapted to excite plasmas for processing very large substrates, for example, substrates used in forming rectangular flat panel displays having sides in the range of 30-100 cm. Excitation of plasmas for treating, i.e., processing, such large substrates requires coils having correspondingly large surface areas in contact with or adjacent a dielectric window structure having a large surface area, commensurate with the areas of the workpieces to be treated. If these prior art structures are used for exciting plasmas for treating large workpieces, numerous problems which apparently have not been previously considered or resolved arise.
A problem common to all of the prior art processor designs is that the windows must be increased to a substantial thickness as the area thereof increases. Otherwise, the windows would not withstand the differential pressure between the atmospheric pressure outside of the chamber and the vacuum in the chamber; e.g. to process workpieces having rectangular treatment surfaces of about 75 cm.times.80 cm, a single quartz window having a surface of approximately 80 cm.times.85 cm must have a thickness in excess of 5 cm. Quartz windows of the stated area and thickness are also very expensive and fragile so use thereof considerably increases the cost of the processor. In addition, we have found that the r.f. fields derived from excitation sources using prior art processor designs are not usually capable of effectively exciting the plasma in a vacuum chamber with a large area, thick window. This is because the r.f. fields do not have sufficient flux density, after penetrating the thick window, to provide the required excitation. For example, the magnetic flux density penetrating a 5 cm thick dielectric window from a coil has a much smaller number of effective magnetic lines of flux than the magnetic field penetrating a 2.5 cm thick window of a prior art device for treating circular wafers having a 20 cm diameter. It is not feasible to simply increase magnetic flux density by increasing current from an r.f. source driving the coil because the increased current can cause excessive heating of the coil as well as other components and because of the difficulty in obtaining suitable high power r.f. sources.
A problem peculiar to the use of prior art induction coils for exciting a plasma having a large surface area is non-uniform excitation of the plasma, resulting in non-uniform plasma density and uneven workpiece processing. We have realized this non-uniform distribution occurs in part because the prior art coils function as transmission lines likely to have lengths, when laid over a large surface window, approaching or exceeding one-eighth wavelength of the r.f. driving sources. Because of the coil length there are significant voltage and current variations along the coil, resulting in appreciable magnetic flux density variations in the plasma. If the coil has a length in excess of one-eighth wavelength of the r.f. source there is an RMS voltage null in a coil driven by a current having an RMS peak value because of the substantial mismatch between the source and the load driven thereby. The mismatch causes the coil voltage and current to be phase displaced by close to 90.degree., resulting in the voltage null. These magnetic flux density variations cause the non-uniform gas excitation and uneven workpiece processing.
We have realized that the length of the coil between terminals thereof connected to the r.f. source must be considerably less than one-eighth of a wavelength of the r.f. source output and that such a result can be achieved by providing a coil with plural parallel branch elements or segments. While Hamamoto et al., U.S. Pat. No. 5,261,962 discloses a planar plasma excitation coil having plural parallel branch segments connected in a ladder configuration to a pair of physically opposed terminals connected to the same ends of leads connected to the branch segments, the structure in Hamamoto et al. is not suitable for use over a large surface area window. If Hamamoto et al. were used on large area windows there would be a tendency for uneven flux distribution and non-uniform plasma density because the different branches are included in r.f. transmission lines with different lengths across the opposed terminals. Hence, the branch segment physically closest to the terminals is in the shortest length line, while the branch segment physically farthest from the terminals is in the longest length line. The different length lines draw different currents from the source so the portion of the plasma adjacent the shortest length line is excited to a considerably greater degree than the plasma portion adjacent the longest length line. This causes non-uniform plasma excitation in processors for treating large surface area workpieces.
It is, accordingly, an object of the present invention to provide a new and improved r.f. field excited plasma processor particularly adapted for treating large workpieces.
A further object of the invention is to provide a new and improved r.f. field excited plasma processor for large workpieces wherein the plasma is uniformly distributed over the workpiece.
Another object of the invention is to provide a new and improved r.f. field excited plasma processor vacuum chamber arrangement particularly adapted for relatively large workpieces wherein dielectric coupling windows are arranged to withstand the differential pressure between the chamber interior and exterior while being thin enough to couple r.f. fields with sufficient density to effectively excite the plasma.
An additional object of the invention is to provide a new and improved r.f. field excited plasma workpiece processor wherein a plasma is inductively excited in an efficient manner to provide relatively uniform plasma distribution for large workpieces.
An added object is to provide a new and improved r.f. field excited plasma processor having plural electrically parallel coil segment branches arranged to supply about the same excitation flux to the plasma.
Yet a further object is to provide a new and improved r.f. field excited plasma processor having plural electrically parallel coil segment branches having about the same electrical and physical lengths to provide uniform flux distribution to the plasma and simplify design of the coil.